Process of manufacturing shaped bodies from iron powders



Patented Mar. 30, 1943 bit? EN T

RCIESS F MANUFACTURENG SMAIED EQDE'ES FRQM HRUN POWDERS No Drawing.Application November 8, 1940, Serial No. 364,797

10 Claims. KCl. 75-22) This invention relates to the production ofshaped sintered bodies from iron powder to the cfiect that apredetermined average content of carbon and. if desired, other additionsis present in the final body. In particular, the invention concerns theproduction by sintering of dense bodies having the character of steel oralloyed steel, from a mixture of different kinds of iron powder.

It was diflicult heretofore, or impossible, to produce by sinteringshaped iron articles of desirable density and carbon content as well asmechanical specifications from powdery initial material.

Powder of steel of any description cannot be compacted by commerciallypracticable pressures into bodies sufficiently coherent for furthermanipulation. Even if pressures of the order of 100 tons per sq. in. ormore are applied, brittle or fragile compacts, if any at all, result.

It was suggested therefore to produce iron powder of desired carboncontent by carburizing ferritic iron powder to desired extent, quenchingthe hot carburized iron powder, crushing and thereafter annealing it inorder to render the particles soft and briquettable'. This process isrelatively expensive and is not satisfactory.

It was also suggested to decarburize steel or other combined carboncontaining iron powder superficially, so as to obtain particlesconsisting of an iron core having retained its carbon content whilesuperficial layers consist of ferritic iron, which is capable of weldingtogether with similarly superficial layers of other particles under highpressure. Upon sintering, however, quite porous bodies are obtained. Theprocess is also expensive and requires great care in its performance.

It is therefore an object of the invention to increase the efiiciencyand lower the cost of production of shaped sintered bodies containing adesired amount of carbon and other additions.

It is another object of the invention to use iron powders containingcombined carbon, in particular steel or cast iron. as available on themarket, without substantially affecting and particularly changing theircarbon content before compacting them under pressure.

It is a further object of the invention to produce shaped sinteredbodies of predetermined average content of carbon and other admixturesfrom two or more kinds of iron powders of difierent carbon content andat least one of which, due to its carbon content and structure,

is normally, 1. e. under pressures up to about tons psi, notbriquettable.

It is a particular object of the invention to produce shaped sinteredbodies from diiferent kinds of iron powders, one of which is normallynot briquettable and contains combined carbon, and if desired, otheradmixtures in predetermined or given amount, while another kind of ironpowder is substantially oxidic and upon reduction after compactingserves to adjust the average carbon content and the bond and structureof the sintered body obtained.

It is still a further object of the invention to produce sintered shapedbodies from two or more kinds of iron powder and a binder, one of thepowders due to its carbon content and structure, being brittle andnormally not briquettable, while another one consists in an iron oxidewhich, upon reduction after compacting serves least in part in thesintered body and consists in the latter case of ferrite powder or anorganic substance.

It is still a further object of the invention to produce by sinteringshaped bodies from three or more kinds of iron powders, one of which issubstantially ferritic or malleable and substantially serves as abinder, another one is of the character of steel, and if desired,alloyed with other admixtures, while a third one acts to adjust theaverage carbon content of the body and adds to the ferritic binderand/or the other kind of iron present in the final body.

It is a particular object of the invention to produce under commerciallypracticable pressures and by sintering shaped iron bodies containing adesired average amount of carbon and, if desired, admixtures as used inalloy steels, from an intimate mixtur of three or more kinds of ironpowders, one of which is substantially ferritic in character ormalleable, another one consists of iron powder containing combinedcarbon and, if desired, other admixtures as used in alloy steels, whilethe third one consists of iron oxide which together with some carbonadded, during sintering operates to adjust the average carbon content ofthe final body and adds to the ferritic binder and/or the other kinds ofiron.

These and other objects of the invention will be more clearly undertsoodwhen the specification proceeds.

Substantially pure or ferritic iron can be produced commercially indifferent ways and is available on the market. Sponge iron, orelectrolytic iron, can be used as such pure iron and crushed to desiredparticle size. However, these kinds of iron are relatively expensive andsometimes difllcult to obtain. It is therefore preferred to manufacturesuch iron powder by desoxidizing iron oxide in well known manner, or

as particularly described in the copending application of Claus GuenterGoetzel, Ser. No. 364,814 and Paul Schwarzkopf, Ser. No. 349,996.

Crushed steel or alloy steel is available on the market. It can beproduced, however, from steel or alloy steel as available on the marketand containing about .1% to 1.5% carbon, by crushing and sieving it and,if particularly fine particle sizes are desired, by repeated crushingand sieving. A steel or alloy steelmay be heated and quenched beforecrushing, if its original brittleness does not sufiice for the intendedpowdering process.

If particular kinds of iron powder are desired containing more carbonthan present in steel grades on the market, cast iron, in particulargray iron containing about 2.4% to 4% carbon or white iron, containingabove about 2% carbon, can be crushed and sieved, or repeatedly crushedand sieved, so as to obtain therefrom a powder of the desired particlesize.

If admixtures, such as tungsten, molybdenum, tantalum, vanadium,titanium, silicon, nickel, cobalt, chromium, and/or manganese aredesired in the final body in an average amount which cannot beintroduced into it by admixture of steel or iron alloys as available onthe market, iron alloys of a desired content of these admixtures are tobe produced, and to be powdered in any of the ways described above forsteel powders.

Such alloys are also available in the market as ferronickel,ferrochrome, ferromanganese, stainless steel scrap, etc. Theseadmixtures can also be added in powdery metallic state.

Iron oxide, particularly in powdery form is available on the market,particularly as mill scale.

According to one feature of the invention, at least three differentkinds of iron powders are admixed, one of the type of ferritic ironpowder, another of the type of iron oxide, and a third of the type ofiron containing admixtures, one of which is always carbon and the othermay be, if desired, molybdenum, tungsten, chromium, nickel, cobalt,vanadium, tantalum, silicon, titanium, manganese, etc. The minimumamount of the ferritic type of iron powder depends on the processing ofthe mixture. The amounts of the other two types of iron powders dependupon the desired ultimate content of carbon and other admixtures in thefinal sintered body.

The powders are intimately mixed, preferably in a ball mill providedwith a steel lining and steel balls of a composition resembling, ifpossible, that of the ultimate product to be obtained by sintering thepowders. Dry milling is generally preferable, though wet milling can beapplied with advantage even though it affords drying of the mixedpowders in a preferably neutral or inert atmosphere, such as desiccatedhydrogen. Ball milling may be performed at room or elevatedtemperatures, up to about 500 to 600 C.

The mixture is then pressed to shape at to 50 tons per sq. inch, butpreferably not exceeding 30 tons, which is commercially practicable.

Due to the presence of the ferritic, soft binder a compact is obtainedwhich can be taken out of the mold or die in which it has been pressed,be subjected to presintering, if desired, and final sintering. If themixture was prepared by hot ball milling so that the particles of harderiron powder, in particular steel have been covered by a thin film offerrite, small amounts of the binder, down to about 5%, are sufficientand permit compacting under pressures from 20 to 30 tons P. S. I. With15% to 20% of the binder and such hot ball milling of the mixture,pressures of 5 to 15 tons P. S. I. sufiice to arrive at a suilicientlycoherent body. If ball milling at room temperature was applied, aminimum amount of ferritic binder from 15% to 20% is preferable forcompacting under commercially practicable pressures. If the steelpowders and particularly the combined iron containing powders wererelatively coarse, say of a size corresponding up to about mesh, aminimum amount of ferritic binder from 25% to 30% is preferable in orderto obtain a sufliciently coherent compact under commercially practicalpressures.

As explained more in detail in the above mentioned copending applicationof Claus Guenter Goetzel, Ser. No. 364,814, filed November 8, 1940, theiron powders containing combined carbon are angular and exhibitsharpedges which, upon being pressed together with soft ferrite, cutinto the latter, and interlock the steel particles with the softferrite.

The ferrite particles are malleable and under pressure therefore adapttheir shapes to those of the other harder iron powders present, so thatthe pressed compact, although still porous, can be subjected to handlingand sintering without losing its shape.

The pressed compact may now be submitted to presintering or immediatelyto final sintering. Presinterlng serves mainly the purpose ofdegasifying the compact, particularly some or entire desoxidation of theiron oxide owder, and to facilitate final sintering. The presinteringtemperature lies about 30% below the melting point of the entire mixtureand can be first established by rough calculation and thereafterverified by a few experiments, and lies between about 750 to 1050 C.

Presintering should be done preferably in an atmosphere neutral or inertto iron, such as desiccated hydrogen, which is capable, however, todesoxidize the iron oxide powder at suitable temperatures, if this bedesired.

For purposes to be described later on in more detail, the use of anatmosphere containing carbon developing gases at presinteringtemperatures, may be advisable; as such atmosphere natural gas (methane)or mixtures of such gases and hydrogen may be mentioned by way ofexample.

The compacted or presintered body is then to be high sintered.

' The sintering temperature depends of course upon the constitution ofthe mixture and particularly its carbon content. It is well known in theart that the melting point of iron decreases with increasing carboncontent, from about 1150 C. for pure iron, to about 1150 C. at 4.2%carbon content, and then increases again slowly with further increasingcarbon content. However, iron with such high carbon content is of nopractical value to date. It is further well known in the art that highsintering of metals ordinarily occurs at temperatures about 10% lowerthan melting temperature and thus the sintering temperature for anymixture of iron powders can be easily established by calculation as wellas a few experiments, for which the calculated average content of carbonof the mix gives a good basis. Ad-

to be used depend entirely upon the composition of the final body to beobtained.

Taking an initial mixture of 20% ferrite powder, 70% powder of steelcontaining 1.5% carbon and iron oxide powder, it is to be consideredthat iron oxide of the formula FeO contains about 30% oxygen and 70%iron. Upon heating the mixture to sintering temperature, the iron oxidedecomposes, 30% oxygen go off and 70% iron remain. Consequently, fromthe 10% iron oxide present in the initial mass, theoretically only 7%reduced iron (ferrite) will remain in the mass. The initial mixture of100% constituents contains therefore after sintering only a total of97%.

The 3% oxygen present in the initial mass combine in part with thehydrogen, if used as atmosphere neutral to iron, and in part with thecarbon introduced by the steel powder into the mass as combined carbon.

The amount of carbon introduced into the mass by the steel powder inthis example, amounted to little more than 1.05% in proportion to theentire mass of iron ferrite, 7% recovered from the iron oxide and 68.95%from the steel) and may be reduced by the iron oxide to about 0.4% to0.5%, calculated upon the entire iron content of the final body.

Thus, upon sintering in desiccated hydrogen a final body will beobtained containing about 0.4 to 0.5% carbon. It may be assumed thatsintering is performed in a push furnace of continuous operation type inwhich the pressed compacts are positioned'on a belt of bendablemolybdenum sheet, and moved on and with the belt through a horizontaltube of nicrom, tungsten or molybdenum which is surrounded over a partof its length by a refractory in which wires or foils of nicrom,molybdenum, tungsten, etc., are embedded and heated to desiredtemperature by an electrical current in an adjustable way. Bycontrolling the amount of hydrogen passed through the tube in the timeunit, by further controlling the speed of the molybdenum sheet andthereby the period of sintering of the compacts thereon, and bycontrolling the sintering temperature, the ultimate amount of carbonremaining in the compact upon desoxidation of the iron oxide can bereadily controlled. The larger the volume per time unit of hydrogenpassed under surpressure through the tube, the larger will be the amountof oxygen taken from the iron oxide and combined with the hydogen andthe smaller the amount of carbon taken by that oxygen from the combinedcarbon present in each individual compact.

Taking the same initial mixture as mentioned above. and assuming that afinal body containing 1% combined carbonis desired, this can be achievedonly if practically none of the combined carbon introduced into themixture by the admixed steel, is burned off (oxidized) during sintering.To this effect the atmosphere in which sintering is performed, shouldcontain carbon developing gases, such as hydro-carbons which decomposeat sintering temperature and give up carbon which can combine with theentire oxygen developed by the iron oxide at sintering temperature.Natural gas (methane) may be used to this effect, and if the carbongiven up by the gas stream passing the tube in a controlled amount pertime unit should be too rich in carbon. It may be diluted with a gassuch as hydrogen. the period and temperature of sintering, and theamount of gas per time unit passing the tube, the desired effect can beobtained.

Instead of using a carbon developing atmosphere, the calculated amountof carbon needed lor desoxidizing the iron oxide contained in theinitial mixture can be intimately admixed to the initial mixturepreferably in the form of lamp black before pressing, sintering is thento be performed in an inert atmosphere, such as desiccated hydrogen.

Taking as another example a final body the carbon content of whichshould be 0.5% in proportion to the entire mass of the iron present, inorder to fabricate gears or tools of desired more or less intricateshape ready for use upon hardening, 20% ferrite powder, 10% iron oxide(pref erably FeO), 30% cast iron (free of silicon and phosphoruscontaining 3.7% combined carbon and 40% steel containing 1.2% carbon maybe admixed, pressed and sintered. Final sintering is to be performed ina hydrogen atmosphere and controlled in the way described above, so thatabout of the oxygen developed by the iron oxide combine with hydrogenwhile of the oxygen serves to reduce the total carbon content of 1.5%(of the original mass) introduced by the cast iron and steel, to thedesired 0.5% carbon content of the resulting total iron mass. The sameeffect can be obtained by sintering in a hydrocarbon containingatmosphere. whereby, however, also a desired carburization of the ironmass can be effected in controllable manner.

Before going more into detail into the operation of the sinteringprocess, the outstanding advantages of this feature of the invention maybe explained. Ferrite powder is used for the purpose of compacting theinitial powder into a coherent body which can easily be taken out of themold in which it was pressed, and subjected to sintering. Ferrite of anyorigin is relatively expensive and its amount should be reducedtherefore to the minimum just necessary for satisfactory compacting'orbriquetting the initial mixture. A higher amount of ferrite is howeveroften desirable in order to arrive at the proper structure of the finalbody. This additional ferrite is introduced according to the inventionby the use of iron oxide power, which isthe least expensive raw materialfor this purpose available on the market, and reduced to ferrite duringthe presintering and/or sintering process in which the use of eitherhydrogen or carbon containing gases is necessary in any event forprotectin the slug undergoing sintering against adverse effects of thesurrounding air. It is diflicult, if possible at all, in commercialproduction to entirely recover that hydrogen, natural gas. etc., it isusually burned off at the discharge end of the furnace and lost.According to the invention, this protective gas is used for the total orpartial desoxidation of the iron oxide admixed to the initial mixture.In' other words, a part of the expensive ferrite which is needed in thefinal sintered product but not for compacting the mixture previous tosintering, is produced according to the invention from non-expensiveiron oxide admixed Here again, by controlling to the initial mixture,during sintering of the shaped mixture, and for this purpose a gas whichin any event has to be used in that sintering process andwhichheretofore was lost at least in part, is utilized for converting theadmixed iron oxide into the desired ferrite.

As it appears from the copending application mentioned above, of C. G.Goetzel, Ser. No. 364,- 814, some ferrite is developed in the iron massof the final sintered body from the steel or other iron containingcombined carbon which has been added to the initial mixture. However,such added steel or cast iron is to be produced from iron oxide in ablast furnace process which is mostly to be followed by refiningprocesses. By introducing the iron oxide immediately into the initialpowdery mixture, expensive conversion processes of the iron oxide aredispensed with. Moreover, iron oxide scrap can b used for this purposewhich is available in the market in vast amounts and at lowest prices.Thus, according to the invention the cost oi the initial mixturesubjected to pressing and sintering is considerably reduced. Though theamount of iron oxide which can be admixed is relatively small, in theexamples given above and generally from about 5% to .40 (resulting inthe first case in a theoretical minimum amount of ferrite in thesintered body of about to and a maximum amount of about 60%), it shouldbe considered that the invention is primarily for mass production inwhich even relatively slight saving in the production of individualobjects count.

It is to be understood that the possible combination of different kindsof iron powder are in a no way exhausted by the above few exemplifica--tions. Thus, instead of steel powder, or a part of it. powder or powdersof alloy steel may be added, if the final sintered body is to containcertain admixtures as usually contained in steel alloys. The dilution ofthose admixtures contained in the alloy steel powder by the mass ofother iron in the final body should be duly taken into consideration.Taking as an example a final body which is to contain 0.2% manganese,and that of an alloy steel powder is introduced into the initial masscontaining manganese, the amount of manganese in that alloy steel shouldbe little less than 0.7%. Such steel alloys can be manufactured in wellknown manner, preferably using a prealloy of ferro-manganese whichcommercially contains from 80% to 82% manganese.

Instead of alloy steel, to the same effect, e. g. white iron can beadmixed which is commercially obtainable with any desired manganesecontent up to about 20%.

Although no theory of the operation of the invention is to be submittedhere, it should be clearly understood that the ferrite powder added tothe initial mixture according to the invention serves primarily forcompacting it into a coherent body which can be handled and sinteredafter pressing and shaping, and that the amount of the ferrite shouldtherefore be limited accordingly. During presintering and in any eventduring sintering. the added'iron oxide is first reduced to ferrite.However, while in ordinary high sintering processes the binder is addedboth for the purpose of agglomerating the other harder particles of themixture during pressing and thereafter during sintering, the ferriteinitially added and subsequently recovered during presintering and/orhigh sintering does not act merely to agglomerate the other harderparticles during sintering. It must be considered that ferrite thoughsofter than the other admixed particles is usually of the highestmelting point of the mixture, while in ordinary agglomerating ent in theadmixed steel, alloy steel or cast iron powders into the ferritepowders, both originally admixed and recovered by reduction of the ironoxide powder added, and thereby the ferrite is partially or entirelyconverted into iron containing combined carbon; upon gradualincorporation.

of carbon into the' ferrite, it is gradually converted into austeniteuntil about 1.7% carbon are absorbed, or into austenite plus cementiteif a larger percentage of carbon is absorbed at high sinteringtemperature. If controlled sintering is performed until all the ferritepresent in the mass is thus converted, eventually a sort of equilibriumwill be attained and the mass will consist throughout of particlescontaining a substantially equal percentual amount of carbon and be ofaustenitic or austenitic and cementitic character. If controlledsintering is carried out only to such an extent that a desired part ofthe ferriteboth initially admixedand recovered from [the reduction ofthe iron oxide, has absorbed carbon from the other powders contained inthe initial mixture, then the mass will eventually consist of thebalance of ferrite which has not absorbed'carbon, of austenite oraustenite plus cementite resulting from the conversion of that ferrite,and of austenite or austenite plus cementite originating from the steel,alloy steel or cast iron powders added to the initial mixture.

In calculating the theoretical sintering temperature of the entire massfrom the sintering temperature of its individual components and theratio in which they are present in the mass. it will appear that thesintering temperature is the higher, the higher the amount of ferriteinitially admixed and recovered from the iron oxide during presinteringand/or sintering, and particularly during heating the mass up to highsintering temperature is.

It will also be found that at such theoretical sintering temperaturesiron powders which contain combined carbon are highly plastified or evenstart to melt. However, the higher the final sintering temperature is,the faster the combined carbon diffuses into the ferrite and therebyreduces its melting and'sintering temperature which is on an averageabout 10% below the melting temperature though this does by no meansform as fixed a value as the melting temperature but includes a rangeclose to but below melting temperatures. It is therefore safe for mostpurposes to work at a high sintering temperature close to thattheoretically calculated for an iron mass containing carbon of an amountintroduced by the iron powders containing combined iron, that mass ofiron to include ferrite and recovered iron. Thus, depending upon thatcombined carbon content calculated on the entire iron mass, the finalsinter-- ing temperature will be within a range of about 1150 to 1390 C.

In this respect as well as regarding the preparation of the mixture fromfine and coarse powders, reference is made to thebroad explanations inthe copending application of C. G. Goetzel, Ser. No. 364,814.

It must be understood that in all these events the carbon content of theinitially admixed steel, alloyed steel or cast iron powders is reducedby the amount of combined carbon which diffused into part or all of theferrite present in the mass during sinteringat high sinteringtemperatures.

It should be further understood that other admixtures than carbon whichwere contained in the admixed alloy steel or cast iron powders, eitherdiffused in part from those powders into the ferrite particles initiallyadmixed and recovered during sintering from the iron oxide, if they arecapable of diffusing into the ferrite at high sintering temperature, asis the case for instance with molybdenum, cobalt, nickel, or remainedsubstantially in those alloy steel or cast iron particles, as is thecase for instance with manganese particularly if present in largepercentage.

It should further be clearly understood that upon cooling the mass thushighly sintered in a controlled manner, the mass undergoestransformation when passing the temperature corresponding to thetransformation point A3 of the well known iron-carbon-temperaturediagram. Depending upon the carbon content of the particles of the massthus cooled, they will result in pearlite if their carbon content at theend of high sintering was 0.9%, or into pearlite plus ferrite, if theircarbon content was lower, or into pearlite plus cementite, if the carboncontent was higher. If cooling down to about 300 C. was performed byparticular quenching, a martensitic structure of the combined carboncontaining particles will result.

The ferrite if retained from that added to the initial mixture andrecovered from the reduction of the iron oxide, will firmly bond theother particles which contain combined carbon. The body will be dense,and certain properties can be developed by subsequent treatment, such asheat treatment of the nature of annealing, and/or mechanical treatment,such as forging, rolling, extruding and drawing. The shaped body canalso be hardened by heating and subsequent quenching, or by casehardening.

According to another feature of the invention, a binder is used which isvolatile entirely or in part. By volatile the invention understands thequality of a liquid or viscous binding fluid to evaporate or espace uponheating to a temperature below high sintering temperature, withoutleaving residues in the mass. In this sense this term is' also used inthe appended claims.

Under partly volatile the invention understands the quality of a liquidor viscous binding fluid to evaporate or escape at or below highsintering temperature, and preferably at or below presinteringtemperature, leaving desirable residues in the mass. In this sense thisterm is also used in the appended claims.

As an example of a volatile binder water may be mentioned.

As examples of a partly volatile binder glycerin, glycol, glucose,dextrine, tar and other liquid or viscous preferably organic substancesor solutions may be mentioned, which upon heating to a few hundreddegrees C. decompose and partly evaporate and partly carbonize.

According to this feature of the invention, ferrite as a binder can bedispensed with entirely or in part.

It has been found that by intimately admixing water to a powdery mixtureof iron oxide and combined carbon containingiron, such as steel, alloysteel, cast iron of any description, a paste is formed which can easilybe pressed in molds to desired shape, under commercially practicablepressures, and upon removing from the mold forms a briquet of sufiicientcohesion to be manipulated and heat treated.

To the same effect admixtures of partly volatile binders can be used.

Thus, into a. powdery mixture of 30% iron oxide and 70% cast ironcontaining 3.7% carbon and corresponding to mesh, water was stirreduntil a paste resulted. The paste was pressed to shape in a mold underhydraulic pressure amounting to about 30 tons P. S. I. A shaped briquetresulted which then was subjected to presintering in hydrogen at atemperature of about 900 C. After about 30 minutes a body resulted whichwas porous but coherent throughout and could bedropped on the floorwithout breaking or crumbling. An analysis showed that practically allthe iron oxide present in the body was converted into ferrite.

Taking into consideration that 30% of iron oxide (FeO) constitutesoxygen, it appears that by the admixture of 30% iron oxide to the mixture, 21% iron and 9% oxygen were introduced into it. This amount ofoxygen would suffice to burn oiT all the carbon introduced into the massby the admixture of cast iron. Therefore, during presinteringdesoxidation of the iron oxide had to be effected mainly by thehydrogen. The oxygen of the oxide also reacted with the carbonintroduced into the mixture by the cast iron, which amounted to 4.2%calculated upon the entire iron mass present in the mixture (21% fromthe iron oxide and 67.41% from the cast iron). The ultimate total carboncontent of the presintered mass was found to be about 1.5%. water was ofcourse entirely evaporated at the start of the presintering process.

The presintered body was then pressed in a die under 30 tons P. S. I.and then sintered at 1300" C. for about one hour, hot pressed andsintered again under the same conditions. A protective atmosphere wasused containing hydrogen and some admixed methane in order to preventdecarburization.

The finally sintered body was dense throughout and exhibitedsatisfactory properties resembling steel of equivalent carbon content.

This feature of the invention offers manifold advantages. Expensiveferrite is entirely dislpensed with. Iron oxide which is the cheapestraw material available is used instead. Desoxidation of the'iron oxidecan be performed easily and as completely as desired during thepresintering step which is always desirable in. powder metallurgy fordegasifying and preshrinking the compact, and during which a porous andwell permeable body is exposed to the action of the desoxi-dizing fluid,such as hydrogen. As iron ipowder containing combined carbon, theequally cheap cast iron can be used for the balance of the mixture, andin spite of the high carbon content of the latter a body can be finallyobtained which resembles steel, because the high carbon The :ontent ofcast iron is also reduced during conrolled presintering.

It should be understood that instead of hydrogen, any other desoxidizinggas can be used such Ls cracked gases, methane, hydro-carbons, which :anbe diluted with hydrogen, etc., so as to reduce n a controlled mannerall or part of the iron oxide iresent during presintering. If desired,part or :omplete desoxidation during presintering can be effected byadmixing solid carbon, such as lamp )lack, in sufficient amount to theinitial mixture and to burn it off during presintering by all or )art ofthe oxygen combined in the iron oxide. in the first case an inert and inthe second case a. reducing or even carburizing atmosphere is ised.

While desoxidation of a porous presintered body s preferable, theinvention is not limited to this. Ihe compacted body taken from the moldcan also be subjected immediately to final high sintering. While thebody is heated to final sintering temperature, the water evaporates orthe partly volatile binder is driven off and decomposed, respectively. Alarge part of or the entire oxygen combined in the oxide will reactduring this heating-up period with the hydrogen, or the carbon containedin the atmosphere used, and before the body has shrunken duringproceeding sintering so far that gases cannot penetrate the bodyanymore, desoxidation will be completed.

During the balance of the sintering period, densi-.

fication of the desoxidized body will be completed.

The fact that iron oxide (Fe-304) melts at a temperature even higherthan FeO and close to the melting temperature of ferrite is particularlyadvantageous. It will stay substantially solid as the admixed cast irondoes, up to high temperatures, thus maintaining the porosity of thebriquet and allowing the hydrogen to penetrate it thoroughly and thegaseous reaction products to escape. Upon completed reduction, the ironoxide is converted into ferrite of slightly lower melting temperatureand the further operation of the sintering process as to diffusion ofcarbon and other admixtures, etc., is the same as described herein withreference to the first feature of the invention.

It appears that iron oxide admixed with water developes qualitiessimilar to those observed in the preparation of slips of ceramic,silicate containing materials. The water forms colloids or compoundswith the oxide capable of uniting the cast iron particles, and upondrying, a coherent body results which can be handled. However, theinventor does not confine himself to any theory of the operation of hisinvention in this respect. In contradistinction hereto, cast iron orsteel powders admixed with water are not briquettable even under farhigher pressures than used in the example mentioned above.

It is understood that instead of cast iron powders, or parts of them,powders of steel, alloy steel, and other iron alloys and metallicadmixtures can be used, of the kind and nature and to the effects asherein-before described more in detail with reference to the firstfeature of the invention, and that the ultimate material, its structureand composition will not differ from that obtained by the use of ferritepowder as a binder, in addition to iron oxide and iron powderscontaining other alloying constituents.

In general, the iron oxide may amount from about to about 70% of themixture. If water is used as a binder, a minimum amount of about to ironoxide is preferable. The pressures for compacting the body should amountfrom about 5 to 50 tons P. S. I., the lower values preferably appliedwhen partly volatile binders are used, while for water as a binderminimum pressures from 15 to 20 tons P. S. I. are preferable, dependingon the particle size. The finer the iron oxide powder, the lower thepressure can be. As to the amount water admixed, it should be such thatupon stirring eventually a thick paste is obtained. Certain limitscannot b given therefor, but in general 2% to 6% by weight of themixture will suffice, considering the low specific weight of water; thelarger the iron oxide amount of the mixture is, the larger should be theamount of water admixed. As to partly volatile binders, an admixture ofa few per cent suffices, about 1% by weight of the mixture as a minimum.

In the appended claims, the terms volatile and partly volatile are usedin the sense defined above, while the term normally briquettable meansbriquettable in the cold under normal or commercially practicablepressures up to about 50 tons P. S. I., and the phrase atmospherecapable of affecting the final average carbon content of the body is tomean gaseous or vaporous fluids of decarburising inert or carburislngeffect upon th compact undergoing sintering, the decarburising fluidsbeing exemplified by normal non-desiccated hydrogen, sometimes dilutedby oxygen derived from decomposition of oxides contained in the compact,while inert fluids are exemplified by desiccated hydrogen andcarburising fluids by natural gases (methane), cracked gases,hydro-carbons and mixtures thereof with hydrogen.

It is to be understood that the invention is not limited to anyparticular exemplification hereinbefore described, but to be derived inits broadest aspects from the appended claims.

What I claim is:

1. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing combined carbon,preferably of the character of steel, comprising the steps of intimatelyadmixing normally not briquettable iron powder containing combinedcarbon with about 5% to powdery iron oxide and a binder, compacting themixture under normal pressure into a coherent body of desired shape, andheat treating the shaped body below its melting temperature undercontrolled conditions effecting reduction of the iron oxide contained inand thereafter final sintering of the body, said conditions includingcontrolled application of temperatures between about 1150 to 1390 C.close to but below the prevailing lowest melting temperature of any ironcomponent contained in said body during final sintering until a denseand strong body of predetermined average carbon content is obtained andpredetermined diffusion of part of combined carbon contained in saidiron powder into iron recovered from said iron oxide is eflected.

2. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing combined carbon,preferably of the character of steel, comprising the steps of intimatelyadmixing normally not briquettable powder of iron containing combinedcarbon with about 5% to 70% powdery iron oxide and solid carbon in anamount sufficient to desoxidise at least part of said iron oxide, andabout 1% to 6% of a binder, compacting the mixture under pressure fromabout 3 to 50 P. S, 1. into a coherent body of desired shape, andheating the shaped body in a protective substantially inert atmosphereso as to first desoxidise at least part of said iron oxide by saidadmixed solid carbon and thereafter finally sinter said body, theteinperatures. of heating not to exceed and at least said final sinterto be effected at temperatures between about 1150 to 1390 C. close tobut below the prevailing lowest melting temperature of any ironcomponent contained in said body until desoxidation of any iron oxide ifstill present is completed, a predetermined diffusion of carbon intoiron recovered from said oxide is efiected and a dense and strong bodyis obtained.

3. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing .1% to 1.5%combined carbon and of the character of steel, comprising the steps ofintimately admixing normally not briquettable .1% to 1.7% combinedcarbon with about 5% to 70% powdery iron oxide and about 1 binder,compacting the mixture under pressure from about 3 to 50 tons P. S. I.into a coherent body of desired shape, Dresintering the shaped body attemperatures between about 750 to about 1050 C., so as to reduce atleast part of said iron oxide, and thereafter heat treating thepresintered body at temperatures between about 1150 C. to 1390 meltingtemperature of any iron component contained in said body in a controlledmanner in an atmosphere capable of affecting the final average carboncontent of the body under treatment in a predetermined way, untilpredetermined difiusion of carbon into iron recovered from said ironoxide is effected and a dense and strong body structurally resemblingsteel and of predetermined average carbon content between .1% to 1.5% isobtained.

4. A method of producing from powdery material .by compacting underpressure and sintering shaped bodies of iron containing combined carbonand of the character of steel, comprising the steps of intimately mixingnormally not bri quettable powder of iron containing combined carbonwith about 5% to 70% powdery iron oxide and solid carbon in an amountsuflicient to desoxidise at least a substantial part of said oxide, andabout 1% to 6% of a binder, compacting said mixture under pressure ofabout 3 to 50 tons P. S. I. into a coherent body of desired shape,presintering the shaped body'at temperatures between about 750 C. toabout 1050 C. until at least part of said iron oxide is desoxidised, andheat treating the presintered body at temperatures between about 1150 C.to 1390 C. close to but below the prevailing lowest melting temperatureof any iron compound contained in said body in an atmosphere capable ofaiTecting the final average carbon content of the body in apredetermined way, until desoxidation of iron oxide if still present iscompleted, predetermined difiusion of combined carbon into ironrecovered from said oxide is effected and a dense and strong bodystructurally resembling steel and of predetermined average carboncontent is obtained.

5. A method of producing from powdery material by compacting underpressure andsintering shaped bodies of iron containing .1% to 1.5%

, combined carbon and of the character of steel,

comprising the steps of intimately admixing normally not briquettablepowder of iron containing .1% to 1.7% combined carbon with 5% to 70%powdery iron oxide and about 5% to 30% of a powder of iron containing C.close to but below the prevailing lowest powdery ferritic binder,compacting the mixture under pressure of about 5 to 50 tons P. S. I.into a coherent body of desired shape, and heat treating the shaped bodybelow its melting temperature under conditions effecting reduction ofthe iron oxide contained in the body and thereafter final sintering ofthe body, said conditions including controlled application oftemperatures between about 1150 C. to 1390 C. close to but below theprevailing lowest melting temperature of any iron component contained insaid body and of an atmosphere capable of aifecting the final averagecarbon content of the body in a predetermined way, until said iron oxideis completely reduced, a predetermined diffusion of carbon into saidferritic binder and iron recovered from said oxide is effected, and adense and strong body structurally resembling steel and of predeterminedaverage carbon content between .1 to 1.5% is obtained.

6. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing combinedcar-bon, preferably of the character of alloy steel, comprising thesteps of intimately admixing normally not briquettable iron powdercontaining combined carbon and other constituents of alloy steel, withabout 5% to powdery iron oxide and a binder, compacting the mixtureunder normal pressure into a coherent body of desired shape, and heattreating the shaped body below its melting temperature under controlledconditions effecting reduction of the iron oxide contained in the bodyand thereafter final sintering of the body, said conditions includingcontrolled application of temperatures between about 1150 C. to 1390 C.close to but below the prevailing lowest melting temperature of any ironcomponent contained in said body during final sintering until a denseand strong body of predetermined average carbon content is obtained andpredetermined difiusion of part of combined carbon contained in saidiron powder into iron recovered from said iron oxide is eifected.

7. In a method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing combined carbon,preferably of the character of alloy steel,

comprising the steps of intimately admixing normally not briquettablepowder of iron containing combined carbon and other constituents ofalloy steel, with 5% to 70% powdery iron oxide and about 1% to 6% of abinder, compacting the mixture under pressure from about 3 to 50 tons P.S. 1. into a coherent body of desired shape, and heat treating in acontrolled manner the shaped body in a carbon discharging atmosphere attemperatures not exceeding about 1000 C. to 1390 C., so as to desoxidisesaid iron oxide, and finally sintering the body at temperatures betweenabout 1150 C. to 1390 C. close to but below the prevailing lowestmelting temperature of any iron component contained in said body in anatmosphere capable of aiiecting to predetermined extent the total carboncontent of the final body, until a dense and strong body ofpredetermined average carbon content is obtained and predetermineddiffusion of carbon into iron recovered from said oxide is eifected.

8. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of the character of alloy steel,comprising the steps of intimately admixing normally not briquettablepowder of iron containing combined carbon and other constituents ofalloy 'steel with 5% to70% powdery iron oxide and tbOllt 5% to 30% ofa'powdery ferritic binder, :ompacting said mixture under pressure ofabout 5 to 50 tons P. S. I. into a coherent body of deaired shape, andheat treating the shaped body oelow its melting temperature underconditions effecting reduction of the iron oxide contained in the bodyand thereafter final sintering of the body, said conditions includingcontrolled application of temperatures between about 1l50 C. to 1390 C.close to but below the prevailing lowest melting temperature of any ironcomponent contained in said body and of an atmosphere capable ofaffecting the final average carbon content of the body in apredetermined way, until said iron oxide is completely reduced, apredetermined diffusion of carbon and other alloying constituentsintosaid ferritic binder and iron recovered from said oxide is effected,and a dense and strong body structurally resembling alloy steel and ofpredetermined average carbon content is obtained.

9. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of iron containing combined carbon,preferably of the character of steel, comprising the steps of intimatelyadmixing normally not briquettable iron powder containing combinedcarbon with about 5% to 70% powdery iron oxide and about 1% to 6% of anat least partly volatile binder, compacting the mixture under normalpressure into a coherent body of desired shape, and heat treating theshaped body below its melting temperature under conditions for drivingoff volatile constituents of said binder, effecting reduction of theiron oxide contained in the body and thereafter final sintering of thebody, said conditions including controlled application of temperaturesbetween about'1150" C. to 1390 C. close to but below the prevailinglowest melting temperature of any iron component contained in said bodyduring final sintering, until a dense and strong body of predeterminedaverage carbon content is obtained and predetermined diffusion of partof combined carbon contained in said iron powder into iron recoveredfrom said iron oxide is effected.

10. A method of producing from powdery material by compacting underpressure and sintering shaped bodies of the character of alloy steel,comprising the steps of intimately admixing normally not briquettablecombined carbon and other constituents of alloy steel with about 5% to70% powdery iron oxide and about 1% to 6% of an at least partly volatilebinder, compacting the mixture under pressure from about 3 to 50 tons P.S. I. into a coherent body of desired shape, and heat treating theshaped body below its melting temperature under conditions for drivingoff volatile constituents of said binder, reducing said iron oxidecontained in the shaped body and thereafter finally sintering it, saidconditions including controlled application of final temperaturesbetween about 1150 C.

to 1390" C. close to but below the prevailing lowest melting temperatureof any iron component contained in said body and of an atmospherecapable of afiecting the average carbon content of the final body in apredetermined manner, until predetermined diffusion of carbon and otheralloying constituents into iron recovered from said oxide is efiectedand a dense and strong body structurally resembling alloy steel isobtained.

RENZO U. VOLTERRA.

powder of iron containing

